• Journal of the Korean Ceramic Society
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• v.39 no.1
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• pp.45-50
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• 2002

Effect of sintering additives on the densification behavior of reaction-bonded silicon nitride prepared by using coarse Si powders is discussed. Sintering additives such as 6 wt% $Y_2O_3$+1wt% $A1_2O_3$ (6YlA) did not give rise to full densification, while full densification was obtained by using the sintering additives such as 6wt% $Y_2O_3$+3 wt% $A1_2O_3$+ 2wt% $SiO_2$ (6Y3A2S) and 9wt% $Y_2O_3$+ 1.5wt% $A1_2O_3$+ 3wt% $SiO_2$ (9Yl.5A3S). In the case of 6Y3A2S addition, high fracture strength of 960 MPa and the fracture toughness of $6.5 MPa.m^{1/2}$ were obtained.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.24 no.10 s.181
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• pp.2628-2636
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• 2000

Densification behavior of mixed copper and tool steel powder under cold compaction- was investigated. By mixing the yield functions proposed by Fleck et al. and by Gurson for pure powder in terms o f volume fractions and contact numbers of Cu powder, new mixed yield functions were employed for densification of powder composites under cold compaction. The constitutive equations were implemented into a finite element program (ABAQUS) to compare with experimental data and with calculated results from the model of Kim et al. for densification of mixed powder under cold isostatic pressing and cold die compaction. Finite element calculations by using the yield functions mixed by contact numbers of Cu powder agreed better with experimental data than those by volume fractions of Cu powder.

• Journal of the Korean Ceramic Society
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• v.37 no.4
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• pp.295-301
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• 2000

The effect of V2O5 addition on the low temperature sintering of Pb(Mn1/3Sb2/3)O3-PZT ceramics, which is known as a prominent material for piezoelectric transformer application was studied, and the densification behavior and piezoelectric characteristics of the samples as a function of heating rate were also examined. V2O5 led the system to liquid phase sintering by forming liquid phase during sintering, which accelerated densification through the particle rearrangement in the early stage of sintering. The liquid phase mostly existed at grain boundaries retarded the evaporation of PbO, while the densification temperature and the weight loss of V2O5-free samples were higher than those of samples with V2O5. Faster heating improved the densification of the samples regardless of V2O5 addition. The low temperature sintering at 100$0^{\circ}C$ was achieved in PMS-PZT ceramics with high density and reasonable dielectric and piezoelectric characteristics. This result revealed optimistic way to the development of multi-layered piezoelectric transformers.

• Korean Journal of Materials Research
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• v.30 no.6
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• pp.301-307
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• 2020

In this study, factors considered to be causes of promotion of densification of sintered pellets identified during phase change are reviewed. As a result, conclusions shown below are obtained for each factor. In order for MA powder to soften, a temperature of 1,000 K or higher is required. In order to confirm the temporary increase in density throughout the sintered pellet, the temperature rise due to heat during phase change was found not to have a significant effect. While examining the thermal expansion using the compressed powder, which stopped densification at a temperature below the MA powder itself, and the phase change temperature, no shrinkage phenomenon contributing to the promotion of densification is observed. The two types of powder made of Ti-silicide through heat treatment are densified only in the high temperature region of 1,000 K or more; it can be estimated that this is the effect of fine grain superplasticity. In the densification of the amorphous powder, the dependence of sintering pressure and the rate of temperature increase are shown. It is thought that the specific densification behavior identified during the phase change of the Ti-37.5 mol.%Si composition MA powder reviewed in this study is the result of the acceleration of the powder deformation by the phase change from non-equilibrium phase to equilibrium phase.

• Journal of Powder Materials
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• v.22 no.1
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• pp.6-9
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• 2015

In this study, the behavior of densification of copper powders during high-pressure torsion (HPT) at room temperature is investigated using the finite element method. The simulation results show that the center of the workpiece is the first to reach the true density of copper during the compressive stage because the pressure is higher at the center than the periphery. Subsequently, whole workpiece reaches true density after compression due to the high pressure. In addition, the effective strain is increased along the radius during torsional stage. After one rotation, the periphery shows that the effective strain is increased up to 25, which is extensive deformation. These high pressure and severe strain do not only play a key role in consolidation of copper powders but also make the matrix harder by grain refinement.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.23 no.11
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• pp.875-879
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• 2010

In this study, we have characterized the roles of $Cr_2O_3$ on the sintering and electrical properties of ZnO. The densification and grain growth of Cr-doped ZnO (ZCr) system was mainly influenced by Cr contents. In the beginning of sintering, the densification of ZnO was retarded as reducing the Zni concentration in ZnO lattice with Cr doping. And the densification and grain growth of ZnO was more retarded due to a formation of spinel phase with increasing the Cr contents. ZCr system revealed varistor behavior with nonlinear coefficient $\alpha$ of 3~23 depending on the sintering temperature, implying double Schottky barrier formation on the grain boundary of ZnO. Especially the best varistor characteristics should be developed with 0.1~0.5 at% Cr contents and under $1100^{\circ}C$ in ZCr systems.

• Journal of the Korean Ceramic Society
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• v.41 no.10 s.269
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• pp.723-727
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• 2004

Three kinds of $Si_3N_4$ powders (M-11, SN-ESP, and SN-E10) were gas-pressure sintered at $1700-1900^{\circ}C$ for 2 h under 18 atm $N_2$. Their densification behavior was investigated and compared as well as the mechanical properties and microstructure of the resulting ceramics. SN-ESP and SN-E10 started to reach nearly full densification at $1750^{\circ}C$ and showed almost no decomposition up to $1900^{\circ}C$. In contrast, M-11 was not fully densified until $1800^{\circ}C$ and showed about $3\%$ weigh loss at $1900^{\circ}C$ indicating poor thermal stability. SN-ESP and SN-E10 showed much higher strength both at room temperature and $1200^{\circ}C$ than M-11 when fully densified. Compared with SN-ESP, SN-E10 was not only a little better in strength (both at room temperature and $1200^{\circ}C$) and fracture toughness but also much higher in the Weibull modulus due to more interlocked microstructure by well elongated grains.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.32 no.5
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• pp.387-395
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• 2008

Stress distribution and interfacial debonding process at the interface between a rubber mold and a powder compact were analyzed during unloading under cold isostatic pressing. The Cap model proposed by Lee and Kim was used for densification behavior of powder based on the parameters involved in the yield function of general Cap model and volumetric strain evolution. Cohesive elements incorporating a bilinear cohesive zone model were also used to simulate interfacial debonding process. The Cap model and the cohesive zone model were implemented into a finite element program (ABAQUS). Densification behavior of powder was investigated under various interface conditions between a rubber mold and a powder compact during loading. The residual tensile stress at the interface was investigated for rubber molds with various elastic moduli under perfect bonding condition. The variations of the elastic energy density of a rubber mold and the maximum principal stress of a powder compact were calculated for several interfacial strengths at the interface during unloading.

• Journal of Powder Materials
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• v.27 no.5
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• pp.373-380
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• 2020

The purpose of this study is to investigate the densification behavior and the corresponding microstructural evolution of tantalum and tantalum-tungsten alloy powders for explosively formed liners. The inherent inhomogeneous microstructures of tantalum manufactured by an ingot metallurgy might degrade the capability of the warhead. Therefore, to overcome such drawbacks, powder metallurgy was incorporated into the near-net shape process in this study. Spark plasma-sintered tantalum and its alloys with finer particle sizes exhibited higher densities and lower grain sizes. However, they were contaminated from the graphite mold during sintering. Higher compaction pressures in die and isostatic compaction techniques also enhanced the sinterability of the tantalum powders; however, a full densification could not be achieved. On the other hand, the powders exhibited full densification after being subjected to hot isostatic pressing over two times. Consequently, it was found that the hot isostatic-pressed tantalum might exhibit a lower grain size and a higher density as compared to those obtained in previous studies.

• Journal of Biosystems Engineering
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• v.26 no.1
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• pp.47-56
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• 2001

In order to prevent the possibility of mixing of metal powder during food grinding processing with the metal roll mill this study was conducted to develope the materials of ceramics roll as a substitute of gray cast iron mill. Since the ceramics is brittle material and can be broken easily by a crack, it was needed to develope engineering ceramics roll materials with high elastic modulus and fracture toughness. Adding 0∼50 wt% Al$_2$O$_3$as densification additives to porcelain body material and forming the ceramics an different condition, mechanical properties were evaluated. The material structure’s densification process was analyzed by SEM and XRD. The evaluation of the mechanical properties of ceramics roll materials were compared and analyzed by non-destructive test using Young’s modulus and destructive test using 3-point bending strength and fracture toughness. The results showed several correlative results. Porcelain body material with 40 wt% Al$_2$O$_3$content heated at 1,200$\^{C}$ for 5h was high bulk density of 2.77, Young’s modulus of 118.4Gpa, 3-point bending strength of 137 MPa and fracture toughness of 2.88 MPa$.$m$\^$$\sfrac{1}{2}$/ . After analyzing the relationship between non-destructive test and destructive test, the coefficient of determination was more than 0.9. Therefore, the evaluation of non-destructive test by ultrasonic was turned out to be feasible in evaluating the mechanical properties of ceramics.